Combustion systems have come under increased scrutiny for toxic emissions which are a by-product of the combustion process. Depending upon the extent of the combustion, carbon monoxide and NOX may be emitted at unacceptable levels. Carbon monoxide levels are normally controlled through complete combustion resulting in carbon dioxide. In fact, in years past, carbon dioxide was measured to determine the efficiency of the process. Traditionally, NOX and other VOC emissions have been either controlled by cleaner fuels or techniques that reduce formation. Recently, because of the push for alternative and renewable fuels, carbon dioxide being the only component here-to-fore not regulated has come under increased scrutiny (green house gases, carbon footprints and global warming).
The problem is, burning fossil fuels makes carbon dioxide, and burning fossil fuels more efficiently makes even more carbon dioxide. NOX and SOX along with other pollutants, which comprise only parts per million in the resulting emissions, are controlled by many effective methods; but there few effective methods of controlling CO2 emissions.
Since CO2 comprises from 10 to 15% of the exhaust or flue gas by volume, it is impractical to treat it without separating it from the remaining gases. Several systems have been developed to reduce the CO2, and in some cases to concentrate the gas. Increasing the amount of hydrogen in the fuel will reduce the fraction of CO2 in the flue gas, since hydrogen combustion does not produce CO2. If the exhaust or flue gas is recirculated and the incoming fuel is mixed with pure O2 the nitrogen in the air is eliminated. And, with enough recycling of the exhaust or flue gas, the CO2 is concentrated to higher levels. After concentration, there are accepted methods of either using or treating the CO2. The burning of the hydrogen simply reduces the CO2. These known methods are quite inflexible and require reconfiguring the combustion equipment. In many cases, very flexible control algorithms will need to be employed to adjust the various fuel-air curves needed for the ever changing fuel compositions. The production or purchase of the hydrogen or oxygen needed in these processes also tend to make them impractical.